The WordPress Transients API makes it easy to save expensive values and look them up later. If you install an object backend (such as APC object cache, memcache or W3 Total Cache) you can cache these values between requests, gaining even more.

But I've noticed that when the name of a transient field is too long, it simply doesn't bother caching it. Aside from being somewhere in the region of 50 characters, I don't know what the maximum length of these names is.

Is this documented somewhere? Does it depend on what backend you're using?